1. Field of the Invention
The present invention relates in general to a method of manufacturing a fluid-filled elastic mount adapted to effect vibration damping based on resonance of a non-compressible fluid through an orifice passage between a pressure-receiving chamber and an equilibrium chamber. More particularly, this invention is concerned with an improved method suitable for manufacturing such a fluid-filled elastic mount whose equilibrium chamber is permitted to undergo a sufficiently large amount of volumetric change and thereby enables the elastic mount to exhibit enhanced damping capability and stability.
2. Discussion of the Prior Art
A cylindrical fluid-filled elastic mount is known as a vibration damping member interposed between two members of a vibration system for flexibly connecting these two members. An example of such a cylindrical fluid-filled elastic mount is disclosed in laid-open Publication Nos. 48-36151 and 52-16554 of unexamined Japanese Patent Applications. The elastic mount disclosed in these publications includes an elastic body interposed between an inner and an outer sleeve for flexibly connecting the two sleeves, and has a pair of fluid chambers which are formed between the inner and outer sleeves in diametrically opposite positions of the sleeves and which are filled with a suitable non-compressible fluid. These two fluid chambers communicate with each other through an orifice passage, so that vibrations applied between the inner and outer sleeves in the diametric direction of the mount may be damped or isolated based on resonance of the fluid flowing through the orifice.
In the cylindrical fluid-filled elastic mount constructed as described above, the elastic body is usually a generally annular member whose inner and outer surfaces are held in contact with substantially the entire circumferences of the inner and outer sleeves. This arrangement tends to cause an excessively large amount of tensile strain to occur in a local portion or portions of the annular elastic body when the elastic mount is installed between two members for flexibly connecting these two members, for instance when the elastic mount is used as an engine mount for flexibly supporting an engine unit on a vehicle body. Namely, the inner or outer sleeve of the installed elastic engine mount receives the weight of the engine unit, as a static load which is permanently applied to the elastic body during the entire life of service for damping a dynamic vibrational load. Therefore, the known fluid-filled elastic mount suffers from insufficient durability of the elastic body.
In the light of the drawback of the known fluid-filled elastic mount described above, an improved arrangement has been proposed as disclosed in laid-open Publication Nos. 62-196434 and 63-289349 of unexamined Japanese Patent Applications. In the proposed improved arrangement, the pair of fluid chambers consist of a pressure-receiving chamber which initially receives a dynamic vibrational load, and a variable-volume equilibrium chamber which is at least partially defined by a flexible diaphragm. More specifically, the pressure-receiving chamber is formed in a portion of the elastic body which is subject to a compressive stress due to the static load. However, no fluid chamber is formed in a portion of the elastic body which is subject to a tensile stress due to the static load. That is, the equilibrium chamber partially defined by the flexible diaphragm need not be defined by any portion of the elastic body, and therefore the elastic body may be located only in a portion of the space between the inner and outer sleeves, which is subject to the compressive stress but not subject to the tensile stress due to the static load. Thus, the elastic body used in the proposed arrangement is substantially free from a tensile strain due to the static load.
However, a study by the present applicants on the above-proposed fluid-filled elastic mount revealed some problems in terms of the durability and damping capability, as described below.
Namely, the volume of the pressure-receiving chamber is reduced when the elastic mount is installed so as to receive the static load, for example, the weight of an engine unit of a motor vehicle, which acts on the outer sleeve. Consequently, the fluid is forced to flow from the pressure-receiving chamber into the equilibrium chamber through the orifice passage, whereby the volume of the equilibrium chamber is increased such that a thin rubber layer or flexible diaphragm partially defining the equilibrium chamber is displaced radially outwardly of the mount. That is, the flexible diaphragm of the installed elastic mount has experienced a certain amount of elastic deformation or displacement due to the static load, prior to receiving a dynamic vibrational load. This condition is undesirable for the durability of the flexible diaphragm, and reduces the maximum amount of elastic deformation of the flexible diaphragm due to the dynamic load. As a result, the maximum amount of volumetric change of the equilibrium chamber, and the fluid flow between the pressure-receiving and equilibrium chambers through the orifice passage are rather limited. Hence, the equilibrium chamber fails to completely attain the assigned function, and the elastic mount is not capable of exhibiting desired vibration damping characteristics, particularly where the amplitude of the input vibrations is relatively large.